Four men in Britain and Ireland who were affiliated with Lulz Security, or LulzSec, a group loosely affiliated with the larger Anonymous hacking collective, were charged with computer crimes, and a fifth man was arrested in Chicago. They are accused of waging attacks against diverse, high-profile targets like Sony and the government of Tunisia.

But the real shocker was that it was LulzSec’s vocal ringleader, Hector Xavier Monsegur — better known by his Twitter handle “The Real Sabu” — who turned them all in. The Federal Bureau of Investigation arrested Mr. Monsegur and charged him with a dozen counts of hacking last August.

Mr. Monsegur proved a rare catch. He was one of the more notorious and brazen hackers in the larger Anonymous movement, promoting and helping to orchestrate attacks against Sony, PBS, Mastercard, PayPal, HBGary and Visa, among others, according to court documents. As the F.B.I. has done with many cybercriminals before Mr. Monsegur, it turned him, and he continued to participate in attacks while providing information about his fellow hackers.

His arrest is likely to put a chill on the hacktivist movement, said Cole Stryker, an author who writes about Anonymous.

“It will be difficult for Anons to work collaboratively now that their ranks are undoubtedly infiltrated by feds, security contractors and rival hackers,” Mr. Stryker said in an interview Tuesday.

Mr. Stryker noted that, in the past, LulzSec and Anonymous attacks have typically required a critical mass of participants to succeed. A so-called distributed denial of service attack, or DDoS attack, in which hackers clog a site with bogus data requests until it crashes, requires that hundreds, if not thousands, of people participate.

That explains why hackers are eager to promote their attacks on Twitter and in the media — it lightens their load. But the more people participate in any given attack, the more likely it is that unsophisticated hackers will not adequately cover their tracks with anonymizing software like Tor, Mr. Stryker noted.

Even sophisticated hackers can make mistakes. The F.B.I. first started tracking Mr. Monsegur after he mistakenly logged into an online chat room without masking his IP address early last year, according to Fox News.

“All it takes is one little slip-up,” Mr. Stryker said. “If one of your layers of obfuscation isn’t there, that’s all it takes for the whole thing to come crashing down.”

In the wake of Tuesday’s arrests, Mr. Stryker said he suspected that would-be Anonymous hackers will think twice before they participate in an attack, for fear they will be exposed.

“It’s likely that Anonymous won’t disappear after these arrests, but will instead revert back to its previous methods of nimble, surprise attacks on unsuspecting victims, committed by hackers who know what they’re doing,” Mr. Stryker said.

Some members of Anonymous disputed the notion that Tuesday’s arrests would have an impact and said hackers would not retreat.

“There are lots and lots of people here that continue to work. The F.B.I. did not really cut the head off of anything. Anonymous will go forward as usual,” said Barrett Brown, who has claimed to speak on behalf of Anonymous in connection with past attacks, like the one on Stratfor in December. “We are prepared for a big slug-out.”

By Tuesday afternoon, Anonymous had already defaced the Web site of PandaSecurity, a firm that hackers accused of aiding the F.B.I. investigation, and replaced it with a video retrospective of its exploits. On Twitter, accounts affiliated with Anonymous called on members to crash the Web site of the F.B.I. with traffic, in retaliation for the arrests.

“Greetings to the spies,” one tweeted. “Just do not forget that we also spy.”